[FEATURE] Add chat:deleteToLineStart action (Ctrl+U clears all input instead of kill-to-beginning-of-line)

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Problem

Ctrl+U in the chat input clears all content instead of performing the standard readline kill-to-beginning-of-line behavior. There is no
chat:deleteToLineStart action available in keybindings.json to customize this.

Impact

Users with iTerm2 configured to send 0x15 (Ctrl+U) on Cmd+Backspace lose the ability to delete the current line. Option+Delete (delete word) works, but there's no way to delete an entire line without clearing all input.

Proposed Solution

Add a chat:deleteToLineStart action that can be bound via keybindings.json, and ideally make Ctrl+U default to this rather than chat:clearInput.

Alternative Solutions

_No response_

Priority

Critical - Blocking my work

Feature Category

Interactive mode (TUI)

Use Case Example

_No response_

Additional Context

_No response_